The unique physical properties of fluorophores allow them to provide optical effects on hair that cannot be matched by typical dyes. For example, the appearance of lightening for dark hair can be offered by optical brighteners. Brassiness in bleached hair can be minimized by fluorescent materials that emit green light which cancels the red components in brassy hair. Fashion shades that are only visible under black light can be achieved with fluorophores that absorb UV but emit visible light. Fluorophores provide many desirable effects on hair that cannot be provided by direct dyes and oxidative dyes alike. Understanding the factors contributing to washfastness would allow us to find ways to enhance the washfastness properties of fluorescent materials.
The alteration of the appearance of keratinous fibers, in particular human hair, by the application of fluorophores has not yet become a common practice in the salon or consumer's homes. However, the number of patent applications and granted patents in this particular field is abundant. According to these publications, applying fluorophores been done mostly with anionic, cationic or zwitterionic fluorescent materials. In some cases, a mercapto group is attached to the fluorophore via a pendant group to allow the fluorescent material to be covalently bonded to keratin proteins to enhance the washfastness of the fluorophore. In some other cases, two fluorophores, typically identical, are linked together by a tethering group to produce a polycationic dimer. The challenge is to still meet all of the other requirements for materials that improve the appearance of hair (e.g., little or no bleeding from the hair when it is wet, evenness, etc.)
We have learned that there are drawbacks in each one of the above approaches to provide consumers an easy and pleasant experience. Anionic and zwitterionic fluorophores are not washfast as they constantly bleed out of hair fibers. Cationic fluorophores are better than anionic and zwitterionic counterparts in bleeding and washfastness, but they would still fade with repetitive use of shampoo for hair cleansing. The approach through covalent bonding via disulfide bonds (reactive fluorophores) does not differentiate proteins in hair from skin. Dimerization of the fluorophores would increase the number of binding sites that minimizes bleeding and loss caused by rinsing by providing stronger hair-fluorophore interactions. However, the same strong binding force to the cuticle also prevents the fluorophores from penetrating deep into the cortex of hair, because it is difficult for fluorescent compounds with multiple positive charges to diffuse through negatively charged networks of keratin proteins. Additionally, since polycationic fluorescent compounds remain bound to the hair surface rather than penetrating into the fiber, it is difficult to produce intense effects due to limited binding sites on the surface of hair. The fluorophores would also be at least twice as big as the monomer, which can become another obstacle for penetration.
Conventional cationic fluorophores do not have much resistance to acid perspiration as they undergo a natural ion exchange process where the cations in human sweat (mainly protons and sodium ions) replace the cationic fluorophores that are deposited on hair. Even washfast fluorophores with multiple cationic anchoring groups have little resistance against a low pH saline solution.